A common point of argument amongst pro and anti-capitalist people is about how much wages people are paid. The thing is, they're both right, in a sense.
So here's the deal: Marx's basic analysis in Capital I assumes that workers are paid a market wage. So let's take a simple problem:
- Market rate for a factory worker is $5/hr.
- It takes 2 hours to make a pair of shoes.
- Shoes can be sold on the market for $15.
So, with this setup, someone who owns a shoe factory can make a profit: the worker $10 for a pair of shoes they can sell for $15.
So, in a sense, they are not under-paying their workers: they are paying the market rate, $5/hr. This is the common pro-capitalist claim with regards to wages. "We pay market rates"
However, because they are making a profit, they're also underpaying, in a different sense: if the workers owned the factory together, they could give themselves a $2.50/hour raise.
Of course, this is just the start of this kind of analysis: a real example has many more details. It's crucial to understand, though, that Marx's analysis of capitalism doesn't inherently rely on owners paying workers less than the market rate. Profit can be made anyway. This is a very intuitive result; everyone knows that there are market rates, people make them, and companies profit. But once you start talking politics, people's judgements get cloudy.